Fuel and oil heat management system for a gas turbine engine

ABSTRACT

A heat management system is provided for a gas turbine engine (10) having first and second oil cooling loops (14, 16). The system distributes excess fuel flow from a main fuel pump (44) among a plurality of upstream locations (58, 60, 68) for managing the transfer of heat between the oil loops (14, 16) and the flowing fuel. A diverter valve (62) regulates the distribution of the bypass fuel responsive to engine heat generation, oil temperature, and/or fuel temperature. A passive fuel distribution configuration using one or more fuel flow restrictors (72, 74, 76) is also disclosed.

FIELD OF THE INVENTION

The present invention relates to a system for transferring heat energybetween the fuel and lubricating oil of a gas turbine engine or thelike.

BACKGROUND

The cooling requirements of gas turbine engines are well known to thedesigners of today's high performance aircraft powerplants. Certaininternal structures, such as bearings, are both cooled and lubricated bya circulating flow of oil which is distributed and collected throughoutthe main engine structure, returning to a central collection point afterhaving absorbed significant heat energy. Another source of heat is theaccessory drive system coupled to the main engine by a mechanical driveand clutch system. Such accessory drives, for example a constant speeddrive for the aircraft service electrical generator, are also providedwith an independent circulating flow of oil for lubricating and coolingpurposes.

One method of cooling the circulating oil loops described above isthrough the use of air-oil coolers and a flow of relatively coolcompressor bleed air. Such coolers, while effective, diminish theoverall engine operating efficiency since the extraction of bleed airincreases overall engine power demand for a given level of usefulthrust. This power penalty results in an increase in engine thrustspecific fuel consumption.

Another method, often used in conjunction with air cooling, is to rejectheat from the circulating oil loops into the flow of fuel entering theengine combustion chamber. This method uses the fuel flow as aconvenient, recuperative heat sink and incurs few of the penalties ofair cooling, but is limited in effectiveness by the maximum temperaturetolerable by the fuel.

In order to appreciate the design problems associated with themanagement of heat generated in these systems, a brief discussion of thefunction and heat output of each is required. Cooling oil circulatingthrough the main engine lubrication system receives heat energy at arate related to the product of engine rotor speed and power output. Thecooling needs of the main engine lubrication loop are thus at a minimumduring periods of low power operation, such as idling, and at a maximumduring high or full power operation, such as takeoff. Normal engineoperation under cruise conditions would fall between the two ranges butcloser to the higher power conditions.

The lubricating and cooling oil of the accessory drive, and particularlyfor an accessory drive provided for the airframe electrical generator,does not receive heat energy proportional to the engine speed and powerlevel but rather as a function of the electrical demand of the airframe.The accessory drive's maximum heat rejection demand may therefore occurat nearly any time in the operation of the aircraft, depending on thenumber of ovens, coffee makers, reading lamps, electrical heaters, orother power consuming devices switched on in the airframe at anyparticular time. The accessory heat rejection demand also varies lessoverall than that of the engine lubrication system, with the minimumheat rate being about one-half of the maximum heat rejection rate.

Against the heat production of the main engine lubrication system andthe accessory drive, the needs of the fuel stream must also beconsidered and balanced. It is typical in gas turbine engineinstallations to deliver the fuel to the engine combustor by a positivedisplacement pump connected mechanically to the rotating engine shaft.It will be appreciated by those skilled in the art that a positivedisplacement pump, such as a gear pump or the like, delivers avolumetric flow rate directly proportional to the speed of the pump. Asthe flow rate from a pump turning proportional to engine shaft speedcould never be made to match the fuel flow requirements of an aircraftgas turbine engine operating under a variety of power level demands andenvironmental conditions, it is common in the industry to size thepositive displacement main fuel pump with an excess flow capacity underall engine operating conditions. The fuel system thus must include afuel control valve and a bypass or return fuel line for routing theexcess main fuel pump output back to the low pressure side of the pump.

The use of a pump bypass, common in many fluid flow applications,normally does not impact the operation of the fuel supply subsystem inan aircraft application. Under certain operating conditions, however,such as engine idling either in flight or on the ground, it will benonetheless apparent that the amount of fresh fuel entering the fuelsystem is small while the relative volume of fuel being bypassed back tothe pump inlet is quite large. The combination of pump inefficiency andrecirculation of the excess main fuel pump output through the bypassline can heat the circulating fuel to an undesirably high temperaturemaking it necessary to provide at least temporary cooling to the fuelsupply system for idle operation.

Various methods have been proposed in the art for accommodating thewidely varying needs of the fuel supply system, main engine lubricationsystem, and the accessory drive unit. U.S. Pat. No. 4,151,710"Lubrication Cooling System for Aircraft Engine Accessory" issued May 1,1979 to Griffin et al, shows disposing the accessory drive fuel-oil heatexchanger downstream with respect to the engine fuel-oil heat exchangerin the fuel supply line. The circulating accessory oil is routed throughor around the accessory fuel-oil heat exchanger and an air-oil cooler inorder to manage the accessory drive heat rejection. The reference alsodiscloses removing heat energy from the fuel stream during periods ofexcessive fuel temperature, such as during ground idle. The total fuelflow passes through both the engine lubrication system fuel-oil coolerand the accessory drive fuel-oil cooler

Such prior art systems, while effective, lack the flexibility forefficiently accommodating the wide variations in heat generationoccurring in the various systems described. In the subject reference,for example, by sizing the accessory fuel-oil cooler to accommodate themaximum mass flow of fuel in the fuel supply line, it is necessary toincrease the size of the accessory fuel-oil heat exchanger so as toaccommodate the higher fuel throughput. Additionally, by placing theaccessory drive heat exchanger downstream of the engine lubricationsystem fuel-oil heat exchanger, the referenced arrangement limits thefuel cooling available to the accessory drive unit, requiring additionalair-oil cooling capacity to achieve current stringent accessory driveoil temperature requirements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system fortransferring heat energy generated in a gas turbine engine among a firstoil loop for cooling an engine accessory drive, a second oil loop forcooling and lubricating the engine bearings and other internalstructures, and the fuel stream supplied to the engine for combustiontherein.

It is further an object of the present invention to distribute said heatenergy responsive to the current rate of heat generation occurringwithin the accessory drive, engine, and fuel stream for achievingefficient and reliable operation over the engine power output range.

It is further an object of the present invention to provide a heattransfer system able to cool the fuel stream by one or more oil loopsduring low power engine operation, and to cool the oil loops with thefuel stream during high power engine operation.

It is still further an object of the present invention to accomplish thedistribution of heat energy by directing a bypass flow of fuel among aplurality of return locations in the fuel stream responsive to thedesired heat transfer performance.

According to the present invention, heat is transferred between each oilloop and a flowing fuel stream by a pair of fuel-oil heat exchangersreceiving the fuel stream in series. The fuel stream passing through thefuel-oil heat exchangers includes at least a portion of the fuelsupplied from the aircraft fuel tank by a boost pump at a metered rateequal to that currently being delivered to the gas turbine enginecombustor.

The fuel stream enters a main fuel pump operating at a fuel flow rate inexcess of the metered rate, hence requiring a portion of the fuelflowing therefrom to be returned to the fuel stream prior to the mainfuel pump. This diversion of the main pump outlet flow is accomplishedby a fuel controller which determines the metered fuel flow rateresponsive to the demanded engine power level.

According to the present invention, a bypass conduit having at least twobranches is provided for returning the bypass flow to two or morelocations in the stream flowing to the main fuel pump, thus altering thefuel flow rate and effectiveness of one or both of the fuel-oil heatexchangers.

The bypass fuel is allocated among the return locations responsive tothe engine power level. Specifically, one embodiment of a systemaccording to the present invention returns the bypass fuel to first andsecond locations disposed respectively upstream of the first loopfuel-oil heat exchanger and intermediate the first and second loopfuel-oil heat exchangers. Allocation of the bypass fuel flow between thefirst and second locations is accomplished by a diverter valvemanipulated responsive to the engine power level.

A second embodiment according to the present invention returns thebypass fuel flow to first and third locations disposed respectivelyupstream of the first loop fuel-oil cooler and downstream of the secondfuel-oil cooler prior to the main fuel pump. Allocation of the bypassfuel between the first and third locations is accomplished passively bythe effect of one or more flow restrictors placed in the bypass returnline. It is an additional feature of this second embodiment that thefresh metered fuel entering the system from the boost pump may bypassthe fuel-oil heat exchangers at high metered fuel flow rates reducingthe total fuel pressure drop between the boost pump and the main fuelpump.

The present invention thus optimally matches fluid temperatures and heatexchange rates between the fuel supplied to the engine and the oil loopsunder all engine operating conditions, thereby reducing the requirementfor auxiliary oil cooling with compressed engine air or the like. Theinvention further provides, for those situations wherein the rate ofheat buildup in the fuel stream is excessive due to a high bypass flowas compared to the metered flow, a means for cooling the recirculatingfuel through a reverse transfer of heat energy from the fuel into thecirculating oil loops.

Still another advantage of the allocating function according to thepresent invention is a reduction in the maximum rate of fuel flowingthrough an individual fuel-oil heat exchanger relative to the minimumrate, thus reducing exchanger size while providing sufficient heattransfer capacity under all cooling conditions. Both these and otheradvantages will be apparent to those skilled in the art upon carefulinspection of the following description and the appended claims anddrawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow schematic of a first embodiment of a fuel and oilheat management system according to the present invention.

FIG. 2 shows a flow schematic of a second embodiment of a fuel and oilheat management system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS

FIG. 1 shows a schematic representation of the fuel and oil flow systemsfor a gas turbine engine 10. An accessory drive 12 is mechanicallylinked (not shown) to the engine 10 and is cooled by a first oil loop 14wherein oil flowing from the accessory drive 12 passes in sequencethrough a first air-oil cooler 18 and a first fuel-oil heat exchanger 20before being returned to the accessory drive unit 12. Cooling air 22,extracted from the compressor or fan section of the engine 10, passesthrough the air-oil cooler 18 and is regulated by a first air controlvalve 24.

Lubricating and cooling oil for the main engine bearings and otherinternal components circulates in a wholly separate oil loop 16, passingin sequence through a second air-oil cooler 26 and a second fuel-oilheat exchanger 28 before returning to the engine 10. Cooling air 30 forthe second air-oil cooler 26 is also extracted from the engine fan orcompressor and is regulated by a second air control valve 32.

Combustion fuel is supplied to the engine from the main fuel tank 34 bya fuel system including an engine driven boost pump 36. Boost pumps aretypically centrifugal pumps designed to operate at an essentiallyconstant pressure for a given engine speed, independent of thevolumetric flow rate of fuel therethrough. Boost pump 36 supplies fuelto a fuel conduit 38 at a flow rate equivalent to the current fueldemand of the gas turbine engine 10. This flow rate, termed the "meteredfuel flow rate", is determined by the main engine fuel control 40 asdiscussed hereinbelow.

The metered fuel flow enters the first fuel oil heat exchanger 20,passing therethrough and flowing subsequently through the secondfuel-oil exchanger 28, a fuel filter 42, and a positive displacementmain fuel pump 44, finally entering the fuel controller 40. It should benoted that the main fuel pump 44 is driven by the engine 10 and thus hasa pump speed proportional to the engine speed.

As discussed in the preceding section, the main fuel pump 44 develops avolumetric flow rate dependent upon the pump shaft speed and istherefore sized to provide a fuel flow at the pump outlet 46 in excessof the metered fuel flow rate. The fuel controller 40 accepts the fuelfrom the pump outlet 46 and divides the flow stream between a supplyline 48 which is routed to the combustion section 50 of the gas turbineengine 10, and a bypass line 52. The fuel flow rate in the supply line48 is the metered fuel flow rate as determined by the fuel controller 40while the fuel flow in the bypass line 52 is equal to the excess mainpump fuel delivery.

In this first embodiment of the present invention, the bypass line 52includes two branches, a first branch 54 and a second branch 56 togetherproviding a means for returning and distributing the bypass flow betweentwo return locations 58, 60, respectively. The first and second returnlocations 58, 60 are disposed respectively upstream of the firstfuel-oil heat exchanger 20, and intermediate the first and secondfuel-oil heat exchangers 20, 28. The flow of bypass fuel is allocatedbetween the locations 58, 60 by a diverter valve 62 operable between afirst position wherein the entire flow of bypass fuel in the bypass line52 is directed to the first return location 58, and a second position(not shown) wherein the entire bypass fuel flow is directed to thesecond location 60. It should be noted at this time that although thediverter valve 62 is disclosed as operating in an either/or fashion fordiverting the entire bypass fuel stream, it may be useful under somecircumstances to employ a partial diverter valve operable for dividingthe bypass fuel between the first and second branches 54, 56 in aproportional manner.

It is preferable to operate the diverter valve 62 responsive to anengine operating parameter related to the rate of heat rejection to theoil loops 14, 16. One such parameter is the fuel pressure rise acrossthe engine driven boost pump 36 which is related to engine speed.

In operation, fuel and oil flow in the above-described systems with heatexchange therebetween accomplished in the fuel-oil heat exchangers 20,28. Under conditions of low engine power, such as idling either on theground or in flight, the metered fuel flow rate is relatively low,matching the fuel demand of the engine 10. As the engine shaft speed atidle is also relatively low as compared to cruise or full power levels,the output of the positive displacement main fuel pump 44, although muchgreater than the metered fuel flow rate, is also reduced. The divertervalve 62 is positioned during these periods to direct the entire bypassfuel flow to the first return location 58 through the first returnbranch 54. In this configuration, the entire bypass fuel flow andmetered fuel flow pass sequentially through the first and secondfuel-oil heat exchangers 20, 28.

During extended periods of idling resulting in excessive heat buildup inthe recirculating fuel, the first fuel-oil heat exchanger 20 acts toremove heat from the fuel by transferring heat in the reverse directioninto the first oil loop 14. This heat is removed from the loop 14 byopening the valve 24 to admit a flow of cooling air 22 through the firstair-oil cooler 18. Similarly, during periods of inflight engineshutdown, heat removed from the windmilling engine, accessory drive, andrecirculating fuel is rejected from the system through the air-oilcoolers 18, 26.

During periods of full power or cruise engine operation, the divertervalve 62 is moved to the second position wherein the entire flow ofbypass fuel is returned to the second return location 60 through thebranch 56. In this configuration, the fresh supply of fuel from the fueltank 34 forms the entire fuel flow through the first fuel-oil heatexchanger 20 wherein the fuel absorbs heat from the circulating oil inthe first loop 14. The second fuel-oil heat exchanger 28 receives boththe bypass fuel returned by the controller 40 as well as the fuelflowing from the first fuel-oil heat exchanger 20. This combined fuelflow passes through the second fuel-oil heat exchanger 28, cooling theoil circulating in the second oil loop 16, and passing subsequentlythrough the filter 42 and main fuel pump 44.

It will be appreciated that during operation at these higher powerlevels, both the metered fuel flow rate and the main fuel pump deliveryrate are considerably higher than those under idle conditions. The highmetered fuel flow rate provides adequate total heat capacity in thesupplied fuel stream for absorbing all the heat energy generated by theaccessory drive 12 and the engine 10 thus allowing closure of the firstand second airflow regulating valves 24, 32 improving overall engineefficiency.

Additionally, by redirecting the bypass fuel return flow from the firstlocation 58 to the second location 60 downstream of the first fuel-oilheat exchanger 20 increases the temperature effectiveness of the firstfuel-oil heat exchanger 20 which receives only fresh fuel from the fueltank 34, unmixed with the warmer bypass fuel stream. This flowconfiguration insures that the maximum cooling capacity of the freshfuel stream is available to the accessory drive unit 12 through thefirst oil cooling loop 14 when the engine operates at full or cruisingpower

One final feature of the embodiment of FIG. 1 are oil bypass lines 64,65 disposed in the oil loops 14, 16 for directing oil around therespective fuel-oil heat exchangers 20, 28. The bypass flows areregulated by control valves 66, 67 which are opened responsive to fueland oil temperature during periods, such as at idle, wherein the fuel istoo hot to absorb additional heat energy, thereby allowing the system tomore flexibly accommodate the needs of the various systems.

By placing the fuel-oil heat exchangers 20, 28 upstream of the main fuelpump 44 and the fuel filter 42, the heat management system according tothe present invention also reduces or eliminates the need for auxiliaryfuel heating to avoid icing up of the fuel filter 42 under extremelycold operating conditions.

FIG. 2 shows a schematic representation of a second embodiment of theheat management system according to the present invention wherein likereference numerals are used to denote elements in common with theembodiment shown in FIG. 1. The second embodiment according to thepresent invention distributes the bypass fuel flowing in bypass line 52between two return locations on the low pressure side of the main fuelpump 44, a first location 58 via a first branch 54, and a third location68, via a third branch 70. It will be appreciated that the returnlocation and branch denoted by reference numerals 68 and 70, whileforming the only other location and branch in the disclosed secondembodiment according to the present invention, are termed the thirdlocation and third branch to distinguish from the second location andsecond branch discussed hereinabove with respect to the firstembodiment.

The second embodiment uses passive means for allocating the bypass fuelflow between the first and third locations 58, 68 comprising one or moreflow restrictors 72, 74, 76, disposed respectively in the first branch54, the fuel inlet of the first fuel-oil heat exchanger 20, and/or thethird branch 70. Based on differential pressures and fuel flow rates atdifferent points in the various fuel lines, the flow restrictors 72, 74,76 allocate not only the bypass fuel flowing in bypass line 52 betweenthe first location 58 and the second location 68, but may additionallyallocate the flow of fresh fuel from the fuel tank 34 between the inletof the first fuel-oil heat exchanger 20 and the second return location68 as discussed hereinbelow.

During periods of low power or idle engine operation when the meteredfuel flow rate is low, bypass fuel in the bypass line 52 flows intobranches 54, 70 and is returned to the supply side of the main fuel pump44 at return locations 58 and 68. During such periods of operation,sufficient flow of recirculating bypass fuel is present through thefirst fuel-oil heat exchanger 20 to permit cooling of the fuel by thefirst oil loop 14 and the first air-oil cooler 18. The exactdistribution of the bypass fuel between the first and second locations58, 68 are determined by the needs of the individual systems, andeffected by sizing the flow restrictors 72, 74, 76 appropriately.

During periods of high engine power operation, such as while cruising orduring takeoff, fresh fuel supplied from the fuel tank 34 is split atlocation 58 between the first fuel-oil heat exchanger 20 and the firstbranch 54. The fresh unmixed fuel bypasses the exchangers 20, 28,joining the bypass fuel in the third branch 70, entering the main fuelpump supply at the third return location 68. The flow restrictors 72,74, 76 are again used to insure a proper distribution of fresh fuelbetween the fuel-oil heat exchangers 20, 28 and the first branch 54according to the heat transfer needs of the joined loops. It should benoted that although the second embodiment is shown in FIG. 2 asutilizing fixed orifice type flow restrictors, it is within the scope ofthe present invention to utilize flow restrictors having different flowcoefficients depending on the direction of the fuel flowing therethroughas well as active fuel flow diverter means such as flow control valvesor the like.

Since the actual sizing and distribution of the recycle and fresh fuelbetween the first and third locations 58, 68 is dependent upon the heattransfer needs of the engine 10 and the accessory drive 12 over theentire engine and drive operating envelope, no specific restrictor sizesor flow proportions are disclosed herein. Such parameters would bedeveloped for each individual engine application based on test results,predicted heat generation rates, required operating environments, andthe specifications of the individual engine manufacturer.

The second embodiment according to the present invention thus reducesthe proportional range of fuel flow rate in both the first fuel-oil heatexchanger 20 and the second fuel-oil heat exchanger 28 by diverting aportion of the fresh fuel from the tank 34 through the first branch 54and third branch 70. The use of flow restrictors 72, 74, 76 to effectthe reversing flow 73 in the first branch 54 provides a passive meansfor allocating the flow of both fresh and bypass fuel between the firstand third return locations 58, 68 over the range of engine operation.

As discussed above with respect to the first embodiment, the highermetered fuel flow rate present at normal engine power levels is morethan sufficient to cool the accessory drive 12 and the engine 10 withoutthe need for diverting cooling air 22, 30 from the engine fan orcompressor sections and thereby avoiding any loss of efficiencyresulting therefrom. It will be appreciated, however, that the coolingair regulating valves 24, 32 may be controlled responsive to the fueland/or oil temperatures in the respective loops 14, 16 as necessary tooptimize system performance over the entire range of engine operation.

The present invention thus provides a heat management system forbenefically distributing fuel in the fuel supply system of a gas turbineengine among various locations with respect to first and second fuel-oilheat exchangers disposed in a heat transfer relationship with the freshand bypass fuel streams for the purpose of maximizing the internal heattransfer between the circulating cooling oil and the fuel. The foregoingdiscussion, while attempting to disclose the invention in broad termscommensurate with the scope thereof, nonetheless has been directed to anexplanation of only two embodiments thereof and should therefore not beinterpreted as limiting, but rather as an illustration of whatapplicants believe is the best mode for carrying out the invention.

We claim:
 1. A system for transferring heat energy among a heatgenerating gas turbine engine, a heat generating accessory drive coupledto the gas turbine engine, a stream of fuel flowing at a metered flowrate, and a stream of cooling air, comprising:a first oil circulationloop wherein a first flow of oil circulates through the accessory drive,a first air-oil cooler having a first, regulated portion of the coolingair stream also passing therethrough, and a first fuel-oil heatexchanger; a second oil circulation loop wherein a second flow of oilcirculates through the gas turbine engine, a second air-oil coolerhaving a second, regulated portion of the cooling air stream alsopassing therethrough, and a second fuel-oil heat exchanger; means forconducting at least a portion of the metered fuel stream, in sequence,through the first fuel-oil heat exchanger, the second fuel-oil heatexchanger, and a main fuel pump, the main fuel pump operating at a fueldelivery rate in excess of the metered fuel flow rate; a fuel controllerfor receiving the fuel flowing from the main fuel pump and dividing thereceived fuel between a supply stream having a flow rate equal to themetered flow rate, and a bypass stream having a flow rate equal to theexcess of the main pump delivery rate over the metered flow rate; andmeans, in fluid communication with the fuel controller and theconducting means, for returning the bypass fuel stream into the fuelconducting means upstream of the main fuel pump at a plurality ofdistinct locations.
 2. The system for transferring heat energy asrecited in claim 1, further comprisingmeans, responsive to an operatingparameter of the gas turbine engine, for apportioning the bypass flowstream among each of the distinct locations in the fuel conductingmeans.
 3. The system for transferring heat energy as recited in claim 1,wherein the plurality of distinct locations includesa first locationdisposed upstream of the first fuel-oil heat exchanger, and a secondlocation disposed intermediate the first and second fuel-oil heatexchangers.
 4. The system for transferring heat energy as recited inclaim 2, wherein the apportioning means comprisesa diverter valve foractively directing the bypass fuel stream among the plurality ofdistinct locations.
 5. The system for transferring heat energy asrecited in claim 2, wherein the plurality of distinct locationsincludesa first location disposed upstream of the first fuel-oil heatexchanger, and a second location disposed intermediate the secondfuel-oil heat exchanger and the main fuel pump.
 6. The system fortransferring heat energy as recited in claim 5, wherein the apportioningmeans includesa flow restrictor disposed in the returning meansintermediate the first and second locations.
 7. The system fortransferring heat energy as recited in claim 6, whereinthe flowrestrictor disposed in the returning means intermediate the first andsecond locations further provides a different coefficient of fluid flowdependent upon the direction of fuel flowing therethrough.